Natural processes involving changes in the Sun could have at least as powerful an effect on global temperature as increased emissions of carbon dioxide (CO2).

Climate scientists have already looked at changes related to Sun spot activity - a cycle of approximately 11 years - and long-term changes in the Sun's brightness, which has a cycle that lasts for centuries.

They have discounted the effect of both on the temperature increase over the last century because they either happen over too short a timescale, or they are too weak.

But so far they have omitted to take two other factors into account:

Changes in the amount of ultraviolet radiation from the Sun affect the ozone layer. This is a very important part of the atmosphere where lots of chemical reactions take place that govern the way the rest of the atmosphere works;

The Sun's magnetic field and solar wind - mainly in the form of electrons and protons coming out of the Sun - protects the entire Solar System by acting as a sort of shield from cosmic rays (very energetic particles and radiation from outer space).

This shield does not stop all the cosmic rays from getting though, and its effectiveness varies with the long-term changes in the activity of the Sun, which can rise and fall on a timescale of centuries.

Cloud cover

One of the effects that cosmic rays have is to influence how cloudy the Earth is.

So if the Sun undergoes long-term changes in activity - which it does - the amount of cosmic rays reaching the Earth will also vary over the same timescale, and so will the planet's overall cloudiness.

The amount of cloud affects the amount of radiation from the Sun reaching the planet surface, which in turn affects the global temperature.

Data collected from satellites show that the amount of low clouds over the Earth closely follows the amount of cosmic rays reaching the Earth.

The resulting warming due to this effect over the last century could be comparable to the amount of warming people think has been due to the greenhouse effect.

Add to that the other effects due to the Sun, and greenhouse gases become less than 50% responsible for rising global temperatures.

Little effect

The other side of this coin is that reducing greenhouse emissions will have much less effect in halting rising temperatures than some people think, and it might have hardly any effect at all.

Our continued use of fossil fuels could make little difference to the climate

The energy emitted from the Sun drives the climate system, and natural changes in its behaviour can have a far greater effect than human behaviour.

Thus, some people may ask: "So why bother worrying about greenhouse gases, and adding billions to the costs of industry to force them to cut emissions, when it could well be a pointless exercise?"

If the Sun is indeed the main contributor to the recent climate change, the money may be better spent providing clean air in big cities and clean drinking water to the Third World.

The author is a solar physicist serving as the European Space Agency's deputy project scientist for the Esa-Nasa Solar and Heliospheric Observatory (Soho)